RWBY-type Free Form
Careful planning isn't really needed. It just depends on your style. I myself run campaigns from scratch with zero prep. Though I find systems very helpful in maintaining consistency, communication, among other things, but for freeform you can simply take notes instead.
Look up The Alexandrian Node Based Design. Really, it makes things much easier for those who do prep, but for those who try to "wing it" many of the concepts there are even more helpful. In the end though, you just need a vague idea of what the important NPCs and their lackeys are doing, then present the consequences to the players.
9 times out of 10, if a GM needs lots of prep, it is because they are making an unfortunately common mistake, which is to have a set plot line that requires the PCs to do or achieve certain things at particular places and times, and much prep work is towards making sure the PCs make the "right" choices to follow the path the GM so painstakingly set up.
All around easier, and better for the players, if the GM sets up a situation and makes the consequences highly visible, and important to, the PCs. Far less prep work, and much easier to build and play "on the fly."